Abstract

We investigated the effect of weather and hydrological variables on the phosphorus concentration in a large, shallow lake, Laguna Chascomús. Total phosphorus (TP) concentrations were high (609.0 ± 196.0 μg L-1) and within the range typical of hypertrophic lakes. TP showed a bimodal seasonal pattern with maxima occurring near spring and fall equinoxes, and the minima around winter and summer solstices. The bulk of TP (>80 %) corresponded to the particulate fraction (i.e., seston). We used data collected during the period 2005-2007 to develop regression models of particulate phosphorus (Ppart) vs. weather and hydrological variables, i.e., incident solar radiation (I0), wind speed (W), mean depth (Z) and water temperature (T). The parameterized models were subsequently used to predict the Ppart concentrations for the 2008 dataset. For developing the first model we took the standard approach of regressing Ppart vs. the independent variables. An alternative to the standard approach was suggested by previous findings from an outdoor experiment demonstrating that increases in incident light resulted in higher seston concentrations, but higher light levels also resulted in lower sestonic P content (Ppart / seston). Thus, for developing the second model we regressed (i) the concentration of seston and (ii) the sestonic P content vs. weather variables, and subsequently calculated Ppart as the product of these two estimates. When confronted against data collected on 2008, the first model failed to explain any significant proportion of the total variance in Ppart (R2 = 0.05, p = 0.28, n = 24), while the second model accounted for 32 % of the variance in Ppart (R2 = 0.32, p = 0.004, n = 24). We suggest that the opposite seasonal trends of seston and Ppart / seston provide a plausible mechanism for explaining the bimodal seasonal pattern of TP observed in this hypertrophic shallow lake.